A global system for mobile communication (GSM) is a radio technology which has been developed as a system for standardizing radio communication systems in Europe, and has widely been deployed all over the world. A general packet radio service (GPRS) is introduced to provide a packet switched data service in a circuit switched data service provided from the GSM. An enhanced data rate for GSM evolution (EDGE) increases a data rate by employing 8-phase shift keying (PSK) instead of Gaussian minimum shift keying (GMSK) employed in the GSM. An enhanced general packet radio service (EGPRS) represents the GPRS using the EDGE.
An enhanced general packet radio service phase 2 (EGPRS2) supporting more various modulation and coding schemes has recently being developed. While the EGPRS supports only two modulation schemes (i.e., GMSK and 8-PSK), the EGPRS2 supports four modulate schemes (i.e., quadrature phase shift keying (QPSK), 8-PSK, 16-quadrature amplitude modulation (QAM), and 32-QAM). By supporting the additional modulation schemes, the EGPRS2 enables a high-speed data service by the use of a great data rate and a high symbol rate.
Hereinafter, the GPRS may include not only the typical GPRS but also the enhanced GPRS (EGPRS)/EGPRS2.
A GSM/GPRS system is based on time division multiple access (TDMA). Information items are transmitted when communication is made between a base station (BS) and a mobile station (MS), and are delivered to the BS or the MS in accordance with a timeslot. Hereinafter, downlink is defined as communication from the BS to the MS, and uplink is defined as communication from the MS to the BS.
When the GSM/GPRS system based on the TDMA is referred to as a 2nd generation (2 G) wireless communication system, a universal mobile telecommunication system (UMTS) based on a wideband code division multiple access (WCDMA) can be referred to as a 3rd generation (3 G) wireless communication system. Standardization on a long term evolution (LTE) wireless communication system based on an orthogonal frequency division multiple access (OFDMA) is also in progress. LTE is also referred to as an evolved-UMTS terrestrial radio access network (E-UTRAN).
With the introduction of various types of wireless communication system, interworking between the existing GSM/GPRS system and a new network (e.g., the UMTS) has arisen as a problem. This is because, when a new network system is introduced, the new network system is preferably compatible with the existing GSM/GPRS system in terms of convenience from the perspective of users and also in terms of the reuse of the existing equipment from the perspective of service provides.
The UMTS supports a compressed mode to measure neighbor cells of a network using a different frequency, which is disclosed in the section 4.4 of 3GPP TS 25.212 V7.1.0 (2006-06) ‘Multiplexing and channel coding (FDD) (Release 7)’. The compressed mode denotes temporary suspension of transmission and reception to perform inter-frequency measurement.
When the MS belongs to the GSM/GPRS cell, a serving cell is the GSM/GPRS cell. A neighbor cell neighboring to the serving cell may be not only the GSM/GPRS cell but also a different network cell. Hereinafter, a different network is defined as a network that uses a different frequency or a different radio access technology from that used in the GSM/GPRS. Examples of the different network may include the UMTS, the E-UTRAN, and a system conforming to the institute of electrical and electronics engineers (IEEE) 802.16 standard. The MS may support a multi-radio access technology (RAT). When the MS supports the multi-RAT, it implies that the MS supports not only the GSM/GPRS but also the different network.
In order for the MS belonging to the GSM/GPRS cell to prepare for a handover to the different network cell which is different from that of the GSM/GPRS system, the MS has to search for the different network cell. The process of searching for the different network cell is performed by the MS to measure a cell of the different network cell or is performed by the MS to attempt to acquire synchronization with the different network cell. In order for the MS to search for the different network cell, radio resources have to be sufficiently ensured. However, it is difficult for the MS to sufficiently ensure the radio resources required to search for the different network cell. This is because a service state has to be maintained with respect to the GSM/GPRS cell until the MS moves to the different network cell.
Accordingly, there is a need for a method capable of searching for a different network cell by an MS while receiving a service from a GSM/GPRS cell, by effectively using limited radio resources.